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Investments - Gross Unrealized Losses and Fair Value for Mortgage-Backed in Unrealized Loss Position (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Less than 12 months |  |
Gross Unrealized Losses | $ (224) | $ (2,752)
Estimated Fair Value | 57,534 | 142,116
12 months or more |  |
Gross Unrealized Losses | (99,116) | (150,916)
Estimated Fair Value | 1,485,532 | 2,123,439
Total |  |
Gross Unrealized Losses | (99,340) | (153,668)
Estimated Fair Value | 1,543,066 | 2,265,555
Mortgage-backed securities |  |
Less than 12 months |  |
Gross Unrealized Losses | (101) | (531)
Estimated Fair Value | 34,601 | 51,391
12 months or more |  |
Gross Unrealized Losses | (54,774) | (61,949)
Estimated Fair Value | 309,842 | 300,318
Total |  |
Gross Unrealized Losses | (54,875) | (62,480)
Estimated Fair Value | 344,443 | 351,709
Utah Housing Corporation bonds |  |
Less than 12 months |  |
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 0 | 0
12 months or more |  |
Gross Unrealized Losses | (318) | (279)
Estimated Fair Value | 2,531 | 3,129
Total |  |
Gross Unrealized Losses | (318) | (279)
Estimated Fair Value | 2,531 | 3,129
U.S. government-sponsored enterprises and Treasuries |  |
Less than 12 months |  |
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 0 | 0
12 months or more |  |
Gross Unrealized Losses | (31,323) | (66,870)
Estimated Fair Value | 966,207 | 1,578,739
Total |  |
Gross Unrealized Losses | (31,323) | (66,870)
Estimated Fair Value | 966,207 | 1,578,739
Other securities |  |
Less than 12 months |  |
Gross Unrealized Losses | (123) | (2,221)
Estimated Fair Value | 22,933 | 90,725
12 months or more |  |
Gross Unrealized Losses | (12,701) | (21,818)
Estimated Fair Value | 206,952 | 241,253
Total |  |
Gross Unrealized Losses | (12,824) | (24,039)
Estimated Fair Value | $ 229,885 | $ 331,978

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